1. Field of the Invention
The present invention relates to a high-frequency circuit which is capable of transmitting or radiating a high-frequency signal in the microwave or millimeter range, and more particularly to a high-frequency circuit capable of exhibiting resonance.
2. Description of the Background Art
In recent years, wireless communication devices have made advancements in terms of downsizing and high-functionalization, which have enabled the drastic prevalence of cellular phones. In the years to come, further downsizing, high-functionalization, and cost reduction are expected.
A high-frequency circuit which is mounted in a wireless communication device such as a cellular phone requires a resonator as an element for composing circuits such as filters, an antenna, and the like.
For example, a ½ wavelength resonator composed of a transmission line whose both ends are open terminated may be used as a resonator. FIG. 25A is an upper plan view showing a conventional ½ wavelength resonator. FIG. 25B is a cross-sectional view of the conventional ½ wavelength resonator shown in FIG. 25A.
A ½ wavelength resonator which is composed of a transmission line 900 whose both ends are open terminated as shown in FIG. 25A needs to be as long as 7.5 cm in the case where its resonance frequency is 2 GHz. Therefore, in order to reduce the circuit size, it is necessary to somehow reduce the resonator length. It is generally known that using a material with high dielectric constant for the circuit substrate 901 can reduce the length of the open-ended transmission line 900, and hence the size of the resonator composed thereof.
On the other hand, it is also generally known that, when a plurality of resonators composed of transmission lines are electromagnetically coupled, the lowest-order resonance frequency thereof can be reduced. FIG. 26A is an upper plan view showing a conventional resonator in which two resonators are electromagnetically coupled together. FIG. 26B is a cross-sectional view of the conventional resonator shown in FIG. 26A composed of two electromagnetically coupled resonators. As disclosed in Document 1 (Microwave Solid State Circuit Design 2nd Edition pp. 275 Wiley-Interscience 2003), if two resonators are coupled together with a short distance between two parallel coupled-lines 902 and 903 contained therein, resonance will no longer occur at a resonance frequency f0 at which resonance would have occurred in the case where there was only a single resonator. Instead, an even mode resonance at a resonance frequency f1 (where f1<f0) and an odd mode resonance at a resonance frequency f2 (where f2>f0) will occur. The more strongly the two resonators are coupled, the farther away the values of f1 and f2 will shift from the value of f0. Therefore, by realizing a stronger coupling between two resonators which have a resonance frequency of f0, a resonator which resonates at a lower resonance frequency f1 (i.e., with a longer wavelength) can be provided; that is, for a given resonance frequency, a resonator having a shorter resonator length can be realized than in the case of employing a single resonator.
However, substrate materials having high dielectric constant are more expensive than substrate materials having low dielectric constant, e.g., resin. Therefore, the aforementioned technique of downsizing a resonator by using a material with high dielectric constant for the circuit substrate leads to cost problems, regardless of whether the entire circuit is formed by using a substrate of a material with high dielectric constant or only the resonator portion is formed of a material with high dielectric constant.
On the other hand, in order to shift the resonance frequencies by introducing a higher degree of coupling between two parallel coupled-lines contained in two resonators, the distance between the parallel lines must be made very short, which means that a drastic improvement in strip formation precision is necessary. However, given the current demands for reducing costs associated with production processes, it is not realistic to improve strip formation precision just for the sake of realizing an extreme reduction in the distance between parallel lines of a resonator. Thus, it would be unrealistic to provide a resonator having a short resonator length by reducing the distance between parallel coupled-lines.
Therefore, what would be practical is to provide a downsized resonator by using a circuit structure which is applicable to a semiconductor process, a production process for a low-temperature sintered ceramic substrate, a multilayer circuit process for a resin substrate, or the like.
It is possible to obtain a high degree of coupling between parallel coupled-lines by deploying two transmission lines in multiple layers, such that the transmission lines overlap each other in the thickness direction. FIG. 27 is a cross-sectional view showing a conventional resonator having an enhanced coupling degree, in which two transmission lines 904 and 905 are disposed in multiple layers so as to overlap each other in the thickness direction. However, the technique illustrating FIG. 27, where two transmission lines are disposed in multiple layers so as to overlap each other in the thickness direction has the following two problems.
A first problem is that there is a limit to the reduction in resonance frequencies that can be achieved based on the capacitance obtained by the parallel overlapping of the two transmission lines 904 and 905. No matter how strong an electromagnetic coupling is obtained by the above technique, the new resonance frequency f1 will not be much below the fundamental frequency f0. This technique is only effective for causing a resonance in the case where the length of the coupled-lines is ½ of the wavelength of the electromagnetic waves. Thus, the length of the coupled-lines is still required to be about ½ of the wavelength, which is a limitation to downsizing.
A second problem is that the resonance obtained from parallel coupled-lines cannot provide adequate spurious prevention characteristics. For example, a band-pass filter used in an actual communication device needs to have not only passing characteristics for a desired band and blocking characteristics for frequencies in the immediate neighborhood of the desired band, but also spurious prevention characteristics for removing harmonic components which may have occurred in various active elements in a previous block. A resonator which is based on parallel coupled-lines is not entirely suitable for use in a communications module since it is impossible to control a resonance which occurs at a frequency which is twice the fundamental frequency.